dm-integrity: fix NULL pointer dereference when the 'R' flag is used

If the dm-integrity device has the SB_FLAG_DIRTY_BITMAP flag set and the
user activates the device in the 'R' mode, a crash in dm_integrity_resume
happens because the function attempts to read the journal containing the
bitmap.

This patch makes dm-integrity skip any writes to the device in
dm_integrity_resume if the device is activated in the 'R' mode.

Signed-off-by: Mikulas Patocka <[email protected]>
Fixes: 468dfca38b ("dm integrity: add a bitmap mode")
Cc: [email protected]

drivers/md/dm-integrity.c

@@ -3875,6 +3875,10 @@ static void dm_integrity_resume(struct dm_target *ti)
 	r = sync_rw_sb(ic, REQ_OP_READ);
 	if (r)
 		dm_integrity_io_error(ic, "reading superblock", r);
+
+	if (ic->mode == 'R')
+		goto skip_writes;
+
 	if ((ic->sb->flags & flags) != flags) {
 		ic->sb->flags |= flags;
 		r = sync_rw_sb(ic, REQ_OP_WRITE | REQ_FUA);
@@ -3984,6 +3988,7 @@ static void dm_integrity_resume(struct dm_target *ti)
 		}
 	}
 
+skip_writes:
 	ic->reboot_notifier.notifier_call = dm_integrity_reboot;
 	ic->reboot_notifier.next = NULL;
 	ic->reboot_notifier.priority = INT_MAX - 1;	/* be notified after md and before hardware drivers */